A
process to upgrade ilmenite to +95% TiO2,
i.e. rutile grade product is described. The process consists
of first oxidizing substantially all the iron values associated with titanium
dioxide to the ferric state (Fe2O3), then reducing the
ferric iron to ferrous state (FeO) to produce a
"synthetic ilmenite" material which is more
reactive than natural ilmenite and amenable to
leaching with 20% HCl at 108-110oC. The
process is capable of upgrading ilmenite from rock as
well as alluvial (sand) deposits, and it permits the ready removal of chromite and other such ferruginous impurities with
negligible loss of titanium oxide. It has no effluent disposal problems and
incorporates well established chemical engineering technology.

Introduction

Tests
have shown that the Murso process can be applied successfully
to all commercial ilmenites whether they be in the form of beach sands or rock deposits. The process
is such that where ferruginous impurities such as chromite
occur in association with ilmenite they can be
removed readily without any significant loss of TiO2 values.

In
the main, commercial ilmenites contain 52-55% TiO2
combined with 40-45% iron oxides. In upgrading ilmenite,
iron oxides ( and oxides of manganese and magnesium
which substitute for iron oxides in the ilmenite
lattice) have to be removed if the required quality is to be obtained.

The Murso Process

The flowsheet of theMurso
process is given in Fig.1. There are five major steps in the process:

1.Oxidation.

2.Reduction.

3.Leaching of the reduced product.

4.Solid-Liquid separation.

5.Recovery and recycling of the hydrochloric acid
from ferrous chloride liquors.

Oxidation

The
iron in ilmenite is present in both the ferrous and
ferric states and the ratio of ferrous to ferric iron varies in different ilmenites and depends upon the degree of alteration of the
mineral. In the Murso process substantially all the
iron values associated with titanium dioxide in the mineral are oxidized to the
ferric state. This places ilmenites from different
localities (and types) and having differing ferrous to ferric ratios, on a
common footing and the process is capable of treating all ilmenites
irrespective of their origin.

Both
oxidation and reduction steps affect the grade of the product and a high grade
(³ 95%) TiO2 is to result. Oxidation is an
exothermic reaction as shown below :

Temperature
of Oxidation

Although
the oxidation of ilmenite starts at temperatures as
low as 250oC, the rate at such low temperature is very slow and does
not become appreciable until a temperature of 800oC is attained. The
oxidation rate increases with rise in temperature.

In
practice, however, the choice of oxidation temperature is determined by
sintering temperature of ilmenite, fuel economy, and
materials of construction for the reactor. While the sintering charachteristics of ilmenites are
dependent on their chemical composition, most ilmenites
start to sinter above 1000oC. Since sintering would decrease the
rate and extentof
oxidation, sub-sintering temperatures are preferred. Furthermore, by operating
below 1000oC materials problems as well as heat requirements are
reduced.

In
view of the above factors, the oxidation step is carried out at a temperature of900-950oC.
Since the heat of reaction is not sufficient to maintain the ilmenite at the oxidation temperature, the heating of the
charge is achieved by fuel injection in the bed with an excess of air to give
approximately 10% oxygen in the products of combustion. The partial pressure of
oxygen required for oxidation is very low and the experimental results show
that the rates of oxidation of ilmenite in air and a
gas containing 10% oxygen are comparable with each other.

In
commercial operations, oxidation is no economic burden since the material would
in any case have to be heated to the reduction temperature (which, in the case
of Murso process is carried out under controlled
conditions of temperature and partial pressure of oxygen).

Fluid
Bed Reactor

Since
ilmenite produced from beach and deposits occurs as a closely sized material
(size range between 100 to 200 microns), it is an ideal material for
fluidization. Thus the steps of oxidation and reduction are preferably carried
out in fluid bed reactors. In continuous operation, both oxidation and
reduction will incorporate two stages to minimize short circuiting of particles
from fluid beds as well as to obtain a maximum utilization of reacting gases
particularly the reductant. In oxidation, the first stage also serves the
function of pre-heating the charge.

Reduction

Preferably
the reduction is carried out in a fluid bed at temperature of 850-900oC
(the process being so controlled that the reduction from ferric to ferrous
state occurs with a minimum formation of metallic iron). This extends further
the process of formation of a large number of sub-grains in the original
ilmenite grain which starts at the oxidation stage and produces a very reactive
"synthetic ilmenite" product. The x-ray diffraction pattern of the
product is similar to original ilmenite but the micro-structure is quite different
and consists of a large number of small grains.

The
reduction of oxidized ilmenite with hydrogen may be represented by the
following reactions :

The
calculated values for the heat of reduction of oxidized ilmenite
show that the partial reduction i.e. ferric to ferrous state,
is slightly exothermic whereas reduction of ilmenite
to metallic iron is endothermic. D.T.A (Differential thermalanalysis) experiments have confirmed
the exothermic nature of the reduction reaction as used in the Murso process. This is a very important consideration when
designing large scale fluid beds as the question of providing heat for
endothermic processes becomes quite critical in fluid beds especially for
reactions where strongly reducing conditions are required. While heat could be
supplied to the bed by heating the incoming reductant
gas, if this is carried out beyond a certain temperature (say above 850oC),
limitations are imposed on the materials forming the hearth of the reactor. However,
where the reaction is exothermic, as is the case above, very little preheating
of the reductant gas is required. The solids from the
oxidizer enter the reducer at a temperature selected for the reduction step.The heat required to bring the reductant
to the reaction temperature is derived from the exothermicity
of the reduction reaction itself. Any heat deficit can be supplied readily by by slight super heating at the oxidation step.

In
those processes necessitating the complete reduction of iron to the metallic
state, temperature over 1000oC are required for kinetic reasons and
even at these temperatures the reaction is thermodynamically very unfavourable. The oxidation step in the Murso
process and limiting the reduction to ferric-ferrous transformation creates
very favourable equilibrium and kinetic conditions.

Reductant

In
view of favourable equilibrium conditions and
kinetics for partial reduction of oxidized ilmenite,
the choice of a reductant in the Murso
process depends upon economic rather than chemical considerations. Since fluid
bed reactors have significant economic advantages ( e.g.
lower capital cost, better heat and mass transfer) over other reactors such as
rotary kiln or multiple hearth furnace, gaseous reductants
are preferred. However, solid reductants are equqlly effective as far as the chemistry of the process is
concerned. Amongst gaseous reductants, hydrogen is
preferred because the rate of reduction below 9000C is much higher
than with carbon monoxide. High purity hydrogen is not necessary and tests have
shown that a gas mixture consisting of hydrogen, carbon monoxide, carbon
dioxide and some water vapour is quite effective.
This type of gas compositon may be economically
produced by the steam reforming of naphtha or natural gas, a typical reductant gas composition being 70% H2, 13% CO2
and 4% water vapour.

Reduction
temperature

As
is the case in the oxidation step, the rate of reduction increases with
increase in temperature but does not become appreciable till a temperature of
700oC is reached. In a commercial process, however, the temperature
of reduction to a certain extent is dependant upon
the temperature of material being discharged from the oxidizer and reduction
temperature 50-100oC below the oxidation temperature is selected. A
typical temperature of reduction is 850oC.

Leaching

Leaching
natural ilmenite in hydrochloric acid has been
proposed but this involves the use of high acid concentrations and temperatures
so that leaching operations have to be carried out under pressure and in two
stages. The reactivity of naturally occurring titaniferrous
ores is so poor that even under above conditions the time required for adequate
leaching is excessive. The design and the materials of construction for the
leaching vessels also present considerable problems and efficient recovery of
32% HCl from chloride containing effluents by
existing commercial processes is considerably more difficult and expensive than
is the recovery 0f 20% HCl.

The
purpose of the leaching is to dissolve selectively iron oxide from ilmenite lattice with a minimum loss of titanium values.
However, some titanium does go into solution and a certain amount of it is
subsequently precipitated from thesolution as a fine material (<
75 microns). The main factors which affect the rate of leaching, titanium
dissolution and its hydrolysis are acid concentration and leaching temperature.
Both high acid concentration and high temperature favour
high rate of leaching. But other factors such as titanium loss in solution,
production of fines, materials of construction for leaching vessels and
economic recovery of hydrochloric acid influence the choice of acid strength
and temperature of leaching.

Taking
the above points into consideration, the optimum conditions for leaching in the
Mursoprocesswere found to
be 20% HCl and temperature of 108-110oC at
atmospheric pressure. In practice, a 20% excess of 20% HCl
over the stoichiometric amount required for leaching
iron is used. Under these conditions, the kinetics of leaching are quite favourable so that batch-wise leaching is complete in 3-4
hours. This is attributed to the formation of "synthetic ilmenite" structure containing many lattice defects,
mainly sub-grain boundaries which greatly enhance the process of diffusion
which seems to control the leaching step. The amount of fines produced is 4-5%
and titanium in leach liquors is less than 1% of the titanium input.

Oxides
of manganese, magnesium, and vanadium which are structurally associated with
TiO2 in the ilmenite lattice are
effectively leached out with iron and partial leaching of aluminium
oxide occurs also. The following table gives typical analyses of ilmenite, murutile, and
commercial rutile.

Analyses of Ilmenite, Murutile,
and Rutile

Heat
of leaching

The
heat associated with the leaching of iron oxide from reduced ilmenite by 20% HCl at 108oC
according to the following reaction has been determined calorimetrically

The
overall heat reaction up to 70-80% extraction was measured to be -21.3 Kcal/gm
atom of iron dissolved. This was in agreement with an estimate of -22 ±
3 Kcal at 25oC using published thermodynamic data.

Since
the leachinf reaction is exothermic, no external
heating is required to maintain the system at the leaching temperature in well
insulated vessels.

Solid-Liquid
Seperation

The
leached solids are classified into a coarse and fine fraction using
conventional equipment (classifier, filter and thickener). The coarse fraction
has a grain size similar to the feed ilmenite. It is
washed, filtered and calcined at 450oC to
remove residual traces of moisture, iron chloride and hydrochloric acid. The
fine fraction is also washed and, preferably, spray dried to the solid state.
This product is suitable for use in the welding electrode industry.

Regeneration
of Hydrochloric Acid

Under
present day conditions, the problem of effluent disposal is vital in
establishing a commercial process. In fact one of the reasons for the change
from the sulphate to the chloride route for producing
TiO2 pigment is the cost and the difficulties associated with the
disposal of ferrous sulphate. The Murso
process has taken into account the important environmental pollution problem
and this one reason for incorporating the step of recovering hydrochloric acid
from ferrous chloride liquors. Ferrous chloride (including manganese, magnesium,
and titanium chlorides) can be easily hydrolysed, and
commercial processes are in operation for the recovery of hydrochloric acid. In
tha main, the steel pickling industry has been
responsible for the development of these processes but they can be easily
extended to ilmenite upgrading system as the problems
are very similar. The regeneration step produces 19-20% HCl
which is recycled and solid iron oxide which may may be used as a feed material in the iron industry,
thus avoiding any effluent disposal problems.

The predominant reaction is :

Most of other metal chlorides present in the spent leach liquor (except calcium) behave similarly.
Excellent recoveries are achieved but a minor amount of make-up acid is required.

Besides
pollution problems, recovery of hydrochloric acid involves economic
considerations also, since under most circumstances it would be cheaper to
regenerate hydrochloric acid than to buy an equivalent amount of acid and then
dispose of the ferrous chloride liquor. It has been estimated by one equipment
manufacturer that the cost of regeneration is approximately £5 per ton (ona 100% HCl
basis) including make up acid cost. The cost includes
capital and operating charges.

Upgrading
Chrome Contaminated Ilmenites

Many
ilmenite deposits are associated with the mineral chromite. Complete separation ofchromite
from ilmenite by magnetic separation is very
difficult because of the similarity in their magnetic susceptibilities. In both
the sulphate acid and the chlorination processes of
pigment manufacture, the presence of relatively small quantities of chromium
mineral in association with the titaniferrous ores is
undesirable. At present ilmenite containing more than
0.1% Cr2O3 is unacceptable by the sulphate
pigment industry and a process to treat ilmenites
containing more than the acceptable Cr2O3 content would
be of great advantage as large tonnage of this material are currently being
dumped.

Murso process is the only process so far reported which
provide a method of converting this hitherto unusable ilmenite
into rutile grade product. Because of the selective
nature of the process chromite and other ferruginous
minerals associated with ilmenite remain virtually un attacked during processing. The clacined
product consists therefore of non-magnetic grains of upgrade together with
magnetic grains of chromite (and other ferruginous
minerals) which can be separated from each other with only insignificant losses
of titanium values by simple magnetic separation.

The Murso process yields an upgrade containing more than 96%
TiO2 and about 0.1% Cr2O3 from an ilmenite, containing as high as 5.5% Cr2O3
with the loss of as little as 1.5% of the TiO2 values.